1. Field of the Invention
This invention relates generally to welding apparatus and methods. In particular, the invention relates to resistance welding preferably of thermoplastic materials.
2. Background Information and Description of the Prior Art
Joining of materials is a critical step in the manufacture of composite products. The recent increase in use of thermoplastic matrix composites, for example in aircraft component parts such as floor panels, stiffeners and skins, presents particular problems with respect to joining of such products. Conventional joining techniques typically involve the use of adhesives or mechanical fasteners. Depending upon the application in which the thermoplastic matrix is to be used, adhesives are often not desirable or acceptable due to unsatisfactory peel properties. Mechanical fasteners can also be undesirable due to the likelihood of cut fibers and stress concentrations. As a result, the concept of fusion bonding or resistance welding has been considered.
In resistance welding, thermal energy is supplied directly to the bond interface between the two pieces of material to be bonded by passing an electric current through a heating element. The heating element is placed between the two pieces of material to be bonded together. The heating element is preferably composed of a unidirectional continuous carbon fiber in a matrix which may be either identical to or different from that of the parts being joined. The joint is heated above the melt temperature or glass transition temperature of the matrix while a consolidation pressure is applied. When uniform melting of the interface is achieved, the current is turned off and the parts are allowed to cool under pressure to assure good bonding. The carbon fibers will be fused directly into the welded sheets of material and are not removed after the welding process.
A problem which has arisen with respect to resistance welding of thermoplastic materials is overheating of the part edges where the heating element enters the lap joint. This causes possible melt through to the part surfaces before welding is completed. This overheating is attributed to the change in the heat transfer mechanism of the heating element from being primarily conductive heat transfer occurring within the weld region to being primarily convective heat transfer occurring in free air. The exposed portion of the heating element is significantly hotter causing localized overheating. This uneven heating results in incomplete welding and poor properties of the welded material.
Another significant problem which has arisen occurs in uneven heating across the length of the heating element. This may be due to inherent small differences in the conductivity of the individual fibers which make up the current path. Alternatively, variations in fiber content or in thickness of the fiber member can cause uneven heating. In addition, a difficulty in achieving identical electrical contact with each of the fibers may also be an explanation for uneven heating. The severity of uneven heating grows with increasing width of the heating element. For example, if a weld joint is large in lateral dimension, a similarly long heating element is used. This gives rise to the problems of uneven heating.
Yet another problem is known as "current leaking" wherein a portion of the welding current is conducted through the laminates being joined rather than solely through the heating element thereby causing a significant loss of heating. This occurs when there is direct electrical contact between (a) the carbon fibers and the heating element; and (b) the fibers of the laminates.
Conventional methods of performing resistance welding include use of an Instron 1125 test machine which is manufactured by Instron Corporation or Cortun, Mass. Also included is a 110 V AC Variac and an inductive ammeter. This equipment utilized a constant displacement process that results in uncontrollable squeeze flow, i.e. fiber movement, that causes significant variation in depth. However, this apparatus does not provide easy control over the consolidation pressure applied to the bond.
U S. Pat. No. 4,871,412 discloses a method of bonding a thermoplastic material to a substrate. The thermoplastic layer comprises carbon fibers coated with polyetheretherketone (referred to herein as "PEEK"). Rollers are used to apply pressure to the layer while current is induced in the layer to weld the layer to the substrate.
U.S Pat. No. 4,054,473 discloses a method for welding a separate strip in overlapping relationship to sections of plastic liners in pipes in order to connect the plastic liner sections. A pair of heater elements serve as electrical conductors. The heater elements have cores which are fused under heated pressure to one side of a welding strip of thermoplastic material. The heater elements are positioned along margins of the strip in spaced and parallel relationship. Current is passed through the heating elements for a period of time sufficient to produce heating and fusing of the heater element core to the pipe liners. This acts to join the plastic liners in the pipes.
A method of spot welding using PEEK and graphite fibers was disclosed in U.S. Pat. No. 4,673,450.
Methods of cooling electrical leads have been known such as that disclosed in U.S. Pat. No. 4,029,837 in the context of magnetic induction coils. U.S. Pat. No. 2,667,437 discloses a coolant which is pumped through openings in electrodes to cool electrodes.
Various methods of using ultrasonic detectors have been known. For example U.S. Pat. No. 4,494,408 discloses a method and apparatus for monitoring and controlling potential residual stress relief mechanisms in composite epoxy resin materials. Ultrasonic energy impinged upon the materials produce signatures identifiable with known residual stress relief mechanisms. U.S. statutory invention registration H465 discloses an ultrasonic method for monitoring resin cure during composite sheet material fabrication.
U.S. Pat. No. 4,950,347 discloses a method of welding thermoplastic resin with a high frequency welder. Higher frequency waves are applied through electrodes to weld the thermoplastic resin. The disclosure is particularly relevant to preparing bags for blood supplies.
There remains a need for an apparatus and method to automate the resistance welding of large lap joints or butt joints in thermoplastic composite panels. There remains a further need to reduce the problems associated with uneven heating across the width of the weld area and overheating of the part edges. There is also need for a nonintrusive method for inspecting and detecting melting at the interface of the weld without resorting to direct temperature methods by way of implanted thermocouples. Furthermore, computer control results in highly repeatable, high quality welds with reduced standard deviation.